placing a sump beside the display does not defeat it's purpose.
a sump is better than HOB
components for a few reasons:
1. it increases the total water volume
2. it allows you to be able to buy a better quality skimmer
3. it makes the display look better because you can put your heaters and stuff in it, and not in the display.
4. you can drain the sump for your water change and not have to stress the fish
5. if you need to add a supplement, you can do so safely, instead of pouring it right in the display and possibly stressing a fish or coral
it might be more expensive in the beginning, but the extra money is small compared to the benefit.